Process for the preparation of a chiral stereorigid metallocene

This is a continuation of our copending application Ser. No. 07/284,918, filed Dec. 15, 1988, and now abandoned.

The present invention relates to a process for the preparation of chiral, stererorigid metallocene compounds of titanium, zirconium and hafnium.

Together with specific cocatalysts, such as, for example, aluminoxanes, chiral, stereorigid metallocene compounds are highly active stereospecific catalysts for the preparation of high isotactic polypropylene. The chiral compounds may be employed in this application as a racemate, but must not be in the meso form since this form does not exhibit stereospcificity.

There is interest in using a compound of the type [(CH 3 ) 2 Si(idenyl) 2 ]ZrCl 2 for polymerization of 1-olefins. However, it has hitherto not been possible to obtain a compound of this type in the form of the pure racemate. In all attempts, a mixture with the meso form has always been obtained.

Catalysts which comprize substituted mono-, di- and tri-cyclopentadienyl coordination complexes of transition metal and aluminoxanes are known (cf. EP-A-129368). They are used for the preparation of polyolefins having a defined molecular weight. Nothing is stated in this publication on the preparation of the transition metal complexes.

It has now been found that silyl-bridged metallocene compounds are obtained in virtually pure racemic form if an alkali metal salt of a silylindenyl compound is reacted at low temperature with a titanium, zirconium or hafnium halide.

The invention thus relates to a process for the preparation of a chiral, stereorigid metallocene compound of the formula I

[R.sup.1 R.sup.2 Si(indenyl).sub.2 ]Y.sup.2 X.sub.2 (I)

in which R 1 and R 2 are identical or different and denot a C 1 - to C 20 alkyl group, a C 2 - to C 20 -alkenyl group, a C 6 - to C 20 -aryl group, a C 7 - to C 20 -alkylaryl group of a C 7 - to C 20 -aralkyl group, Y 2 is titanium, zirconium or hafnium, and X denotes a halogen atom, which comprizes reacting a compound of the formula (II),

[R.sup.1 R.sup.2 si(indenyl).sub.2 ]Y.sub.2.sup.1 (II)

in which R 1 and R 2 have the abovementioned meaning and Y 1 is an alkali metal, with a compound of the formula III

Y.sup.2 X.sub.4 (III)

in which Y 2 and X have the abovementioned meaning, at a temperature of from -78° C. to +25° C. in an inert solvent for 10 to 1000 minutes.

For the process according to the invention, a silyl compound of the formula II

[R.sup.1 R.sup.2 Si(indenyl).sub.2 Y.sub.2.sup.1 (II)

is employed. In this formula, Y 1 is an alkali metal, preferably lithium. R 1 and R 2 are identical or different and denote C 1 - to C 20 -, preferably C 1 - to C 6 -alkyl, C 2 - to C 20 -, preferably C 2 - to C 6 -alkenyl, C 6 - to C 20 - preferably C 6 - to C 12 -aryl, C 7 - to C 20 -, preferably C 7 - to C 10 -alkylaryl, C 7 - to C 20 -, preferably C 7 - to C 10 -aralkyl, and, in particular, R 1 and R 2 denote methyl or phenyl. Li 2 [(CH 3 ) 2 Si(indenyl) 2 ] is particularly preferably employed.

The compound of the formula II is reacted with the compound of the formula II

Y.sub.2 X.sub.4 (III)

in which Y 2 is titanium, zirconium or hafnium, preferably zirconium or hafnium, and X is a halogen atom, preferably chlorine. These metal halides are used either as such or as solvate complexes, for example with tetrahydrofuran. The solvents must be free from moisture and oxygen.

The reaction is carried out in an inert solvent. Suitable solvents are aromatic hydrocarbons, such as, for example, toluene or xylene, aliphatic hydrocarbons, such as, for example hexane or pentane, or ethers, such as, for example, diethyl ether, tetrahydrofuran or dioxane. Toluene, diethyl ether and tetrahydrofuran are preferably used.

The reaction temperature is 31 78° C. to +25° C., preferably -40° C. to 0° C.

The reaction time is 10 to 1000, preferably 60 to 180, minutes.

The reaction is carried out with stirring and in an inert-gas atmosphere.

The ansa-metallocene compounds of the formula I

[R.sup.1 R.sup.2 Si(indenyl).sub.2 ]Y.sup.2 X.sub.2 (I)

thus obtained in which R 1 , R 2 , Y 2 and X have the abovementioned meaning, can be recrystallized from aliphatic and aromatic hydrocarbons and etheral or halogen-containing solvents. Mixtures of chloroform and hexane have proven particularly favorable.

The process according to the invention has the advantage that the silyl-bridged metallocene compounds are producted in racemic form and contain very low amounts of the meso form, or none at all. This is of decisive importance, particularly when the complexes are used as catalysts, such as, for example, in the polymerization of α-olefins, since only the racemic form, but not the meso form, exhibits stereoselectively. Whereas, for example, rac-[C 2 H 4 (indenyl) 2 ]Ticl 2 , produces highly isotactic polypropylene, its meso form only produces atactic polymer.

All the operations below were carried out in an inert-gas atmosphere using absolute solvents.